The invention relates to a system for transmission, by the modulation of a carrier, of a train of unipolar signals, such as facsimile signal or similar data signal, which alternately assume a definite voltage level, e.g., a black level or mark level, and an arbitrary voltage level of one polarity which differs from the definite level; and more particularly, the invention relates to an efficient transmission system capable of transmission with a transmission bandwidth which is less than that required for the normal amplitude modulated double sideband transmission system.
Heretofore, a facsimile signal or a similar unipolar signal has been transmitted by the amplitude modulated double sideband transmission system or frequency modulation system. These modulation systems require a transmission frequency bandwidth which is at least twice the frequency bandwidth of the original signal. A band compression modulation system is also known in which the original signal is converted into a multi-level signal to provide the band compression, followed by an amplitude or frequency modulation with the multi-level signal, thus forming the multi-level modulation system. Because of the multi-level amplitude, this system is susceptible to the influence of any degradation in signal to noise ratio or the waveform distortion over the transmission path, thereby degrading the received waveform. A vestigial sideband transmission system is also known as another band compression modulation system. In this system, filter means is required for shaping the vestigial sideband frequency characteristic, and also a carrier re-generator for synchronous detection is required on the receiving side, both of which add to the complexity of the overall system.
U.S. Pat. No. 3,761,610 issued to R. E. Krallinger et al and entitled High Speed Facsimile Systems discloses the conversion of a facsimile signal into a three level signal to reduce the bandwidth to nearly one-half, and the latter signal is used to provide a balanced modulation of a carrier, the modulation output being transmitted in the form of a vestigial sideband signal. On the receiving side, the carrier is regenerated for the purpose of synchronous detection of the received signal to demodulate it into the three-level signal, which is then converted into the two-level signal, thus deriving the original signal. In this system, the frequency bandwidth is reduced to about one-half that of the original signal as a result of the conversion into the three level signal, and additionally the use of the vestigial sideband transmission further compresses frequency bandwidth. However a converter for conversion into the three level signal must be provided together with a filter for shaping the vestigial sideband frequency characteristic. Also on the receiving side, there must be some means for extracting the carrier and the synchronous detector as well as another converter for converting the three level signal into the two level signal, thus adding the complexity of the overall arrangement.
It is an object of the invention to provide an efficient transmission system of a relatively simple arrangement which requires less transmission bandwidth than the amplitude modulated double sideband transmission system.
It is another object of the invention to provide an efficient transmission system having a simplified arrangement for the modulator and the demodulator and which achieves a comparable band compression effect as that obtained when the signal is converted into the multi-level signal.
It is a further object of the invention to provide an efficient transmission system which eliminates the conversion into the multi-level signal and thus is insensitive to the influence of level fluctuations and noises than the transmission involving the multi-level conversion, and which still achieves the comparable band compression effect as the latter.
It is still another object of the invention to provide an efficient transmission system which enables the use of a receiver circuit of the same type as a conventional receiver used for the amplitude modulation system employing the envelope detection and which is compatible with the conventional amplitude modulated double sideband transmission system.
It is a still further object of the invention to provide an efficient transmission system which enables the original signal to be reconstructed, by utilizing a phase information, even when the waveform of the envelope is substantially distorted in the transmission path.
It is an additional object of the invention to provide an efficient system which lends itself to construction with a digital circuit.
It is still additional object of the invention to provide a transmission system of an increased efficiency which is achieved by combination with either the multi-level conversion or the vestigial sideband transmission.